The present invention relates to a modulated intensity output solid state laser and, more particularly, to a modulated intensity output vertical cavity surface emitting solid state laser.
Large numbers of closely spaced lateral circuit interconnections, extending between various portions of individual integrated circuit chips, between various integrated circuit chips mounted on a printed circuit board, and between various printed circuit boards mounted in a system, that can each transmit large numbers of signal symbols with extreme rapidity are increasingly needed. These interconnections are needed to move, between selected locations, the large amounts of data generated by very fast signal processors that appear on signal busses for transmitting signal symbols representing such data, data that is to be received and sent by those processors and by various related data receiving, using, generating and transmitting devices.
As chip area and board mounted component density increases, the numbers of unavoidable, but unwanted, electrical circuit couplings, or parasitics, will most certainly increase substantially. Dynamic power dissipation in on-chip and off-chip circuits for operating circuit interconnections comprises the vast majority of total power consumed. Dynamic dissipation scales linearly with switching speed, and so power consumption per line in electrical interconnections can be expected to soon outstrip that of their optical interconnection counterparts where the power dissipation is essentially independent of signal path length over those interconnections. Hence, there will be transitions in the future to optical interconnection based system architectures.
These optical interconnection arrangements will require low cost, low power, directly modulated, high-reliability, single-chip laser sources and source arrays operating at data rates in excess of 17 Gbps, now, but capable of reaching 100 Gbps in the future, to meet the demands of existing, and emerging future, serial chip and board data communications requirements. Such required capabilities for the laser sources lead to difficult requirements to be met by those sources in terms of power dissipation, reliability, and interconnection spatial densities.
Single lasers and one dimensional and two-dimensional laser arrays are needed for fiber optic links, board-to-board and chip-to-chip links. Each laser should dissipate less than 2 to 5 mW/laser. Reliability must be greater than 100,000 hours (10 years) at a minimum. Device-to-device uniformity needs to be high (variations being less than 5%), and device aging characteristics must be sufficiently slow to eliminate any need for power monitoring. Low device lasing thresholds and high modulation efficiencies will be required to minimize electrical power drains in the laser driver arrays. In addition, in the case of intra-chip optical interconnects, thermal dissipations pose a particularly challenging problem as the components may be expected to operate at ambient temperatures in excess of 80 C. This not only will have a significant impact on device intrinsic bandwidth, but on device reliability as well.
Vertical cavity surface emitting lasers (VCSELs) have been found to be suitable laser sources for short transmission distance optical networks with 10 Gbps VCSELs being the laser devices with the largest modulation rates commercially available today. VCSELs thus are the dominant light emission source for short transmission distance optical interconnection arrangements and local area networks because of their large modulation rate capabilities, low power consumption, spatially dense device array integration, and low cost manufacturing of those devices when made in sufficiently large numbers.
VCSEL sources that are directly modulated to correspondingly vary the emitted light intensity at large modulation rates offer a substantial decrease in cost over the typical alternative, a CW laser operated in conjunction with an external adjacent electro-absorption modulator. An important figure of merit for modulation rates in lasers is the −3 dB small-signal modulation bandwidth that is defined as the point at which the modulated optical output, measured as a function of frequency, is reduced to half of its low modulation rate value. A variety of methods have been used to achieve greater modulation rates of light intensities emitted by VCSELs. These have included use of metal contacts on polymer layers as well as ion implantation to reduce device capacitance to achieve small-signal modulation bandwidths of 16 to 20 GHz. Although state-of-art VCSELs in laboratories have been demonstrated to provide modulation bandwidths of 40 Gbps, current VCSEL technology makes achieving modulation bandwidths greater than 10 Gbps in practice very difficult because of reduced device reliability if operated at the large current densities required to do so. Therefore, VCSELs that can be reliably operated with greater modulation bandwidths are desired.